1. Field of the Invention
This invention relates to a creep preventing device for vehicles equipped with an automatic transmission.
2. Description of the Prior Art
With a speed change lever being set in the drive position (forward-movement position) during a stop, vehicles equipped with an automatic transmission present the so-called creep phenomenon that the vehicle tends to move forward against an intention of the driver due to the dragging torque of a torque converter. Such a creep force is going to apply a brake force to the engine during the idle operation. To hold the number of revolutions of the engine during the idle operation at a reference value, therefore, it is required to increase the opening degree of a throttle valve during the idle operation to a certain extent for compensating the brake force. This increases fuel consumption as a matter of course and was one of primary causes to increase the fuel cost of vehicles equipped with an automatic transmission.
In veiw of the above, the assignee of the present invention has previously proposed an improvement in that the idle operation state of a vehicle is detected and the pressure exerting on a starting clutch is bypassed to an oil tank to interrupt power transmission between the engine and driving wheels, so that the load upon the engine becomes the same level as that in vehicles equipped with a manual transmission. Such an improvement has, however, accompanied a drawback as follows. Since the starting clutch is controlled to reengage based on the digital value of ON/OFF type when an accelerator pedal is trod on, some shock causes unavoidably at the time of reengagement and hence it is difficult to ensure the smooth starting operation from the standpoint of mass production. Such starting shock can be alleviated by provision of various gradually increasing pressure mechanisms, e.g., accumulator, which can moderately increase the rising speed of oil pressure when the starting clutch is reengaged. If an accumulator is arranged to actuate when the accelerator pedal is trod on, there tends to cause a time lag at the time of quick starting. This results in a limitation that capacity of the accumulator can not be set so large. Thus, the above improvement could not offer the basic solution.